Methods and apparatus for three-dimensional nonvolatile memory

ABSTRACT

A method is provided that includes forming a dielectric material above a substrate, forming a hole in the dielectric material, the hole disposed in a first direction, forming a word line layer above the substrate via the hole, the word line layer disposed in a second direction perpendicular to the first direction, the word line layer including a first conductive material having a first work function, forming a nonvolatile memory material on a sidewall of the hole, the nonvolatile memory material including a semiconductor material layer and a conductive oxide material layer, forming a local bit line in the hole, the local bit line including a second conductive material having a second work function, wherein the first work function is greater than the second work function, and forming a memory cell comprising the nonvolatile memory material at an intersection of the local bit line and the word line layer.

BACKGROUND

Semiconductor memory is widely used in various electronic devices suchas mobile computing devices, mobile phones, solid-state drives, digitalcameras, personal digital assistants, medical electronics, servers, andnon-mobile computing devices. Semiconductor memory may includenon-volatile memory or volatile memory. A non-volatile memory deviceallows information to be stored or retained even when the non-volatilememory device is not connected to a power source.

One example of non-volatile memory uses memory cells that includereversible resistance-switching memory elements that may be set toeither a low resistance state or a high resistance state. The memorycells may be individually connected between first and second conductors(e.g., a bit line electrode and a word line electrode). The state ofsuch a memory cell is typically changed by proper voltages being placedon the first and second conductors.

In recent years, non-volatile memory devices have been scaled to reducethe cost per bit. However, as process geometries shrink, many design andprocess challenges are presented

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts an embodiment of a memory system and a host.

FIG. 1B depicts an embodiment of memory core control circuits.

FIG. 1C depicts an embodiment of a memory core.

FIG. 1D depicts an embodiment of a memory bay.

FIG. 1E depicts an embodiment of a memory block.

FIG. 1F depicts another embodiment of a memory bay.

FIG. 2A depicts an embodiment of a portion of a monolithicthree-dimensional memory array.

FIG. 2B depicts an embodiment of a portion of a monolithicthree-dimensional memory array that includes vertical strips of anon-volatile memory material.

FIGS. 3A-3E depict various views of an embodiment monolithicthree-dimensional memory array.

FIGS. 4A1-4J2 are cross-sectional views of a portion of a substrateduring an example fabrication of the monolithic three-dimensional memoryarray of FIGS. 3A-3E.

DETAILED DESCRIPTION

Technology is described for forming monolithic three-dimensionalnonvolatile memory arrays. In particular, technology is described forforming monolithic three-dimensional nonvolatile memory arrays thatinclude reversible resistance-switching memory cells that includeresistance-switching elements that include a semiconductor materiallayer and a conductive oxide material layer. Each reversibleresistance-switching element is disposed between a word line and a bitline. The word line includes a first conductive material having a firstwork function, the bit line includes a second conductive material havinga second work function, and the first work function is greater than thesecond work function.

An example method includes forming a dielectric material above asubstrate, forming a hole in the dielectric material, the hole disposedin a first direction, forming a word line layer above the substrate viathe hole, the word line layer disposed in a second directionperpendicular to the first direction, forming a nonvolatile memorymaterial on a sidewall of the hole, forming a local bit line in thehole, and forming a memory cell including the nonvolatile memorymaterial at an intersection of the local bit line and the word linelayer. In an embodiment, the word line layer includes a first conductivematerial having a first work function, and the local bit line includes asecond conductive material having a second work function, wherein thefirst work function is greater than the second work function.

In some embodiments, a memory array may include a cross-point memoryarray. A cross-point memory array may refer to a memory array in whichtwo-terminal memory cells are placed at the intersections of a first setof control lines (e.g., word lines) arranged in a first direction and asecond set of control lines (e.g., bit lines) arranged in a seconddirection perpendicular to the first direction. The two-terminal memorycells may include a reversible resistance-switching memory elementdisposed between first and second conductors. Example reversibleresistance-switching memory elements include a phase change material, aferroelectric material, a metal oxide (e.g., hafnium oxide), a barriermodulated switching structure, or other similar reversibleresistance-switching memory elements.

Example barrier modulated switching structures include a semiconductormaterial layer adjacent a conductive oxide material layer (e.g., anamorphous silicon layer adjacent a crystalline titanium oxide layer).Other example barrier modulated switching structures include a thin(e.g., less than about 2 nm) barrier oxide material disposed between thesemiconductor material layer and the conductive oxide material layer(e.g., an aluminum oxide layer disposed between an amorphous siliconlayer and a crystalline titanium oxide layer). As used herein, a memorycell that includes a barrier modulated switching structure is referredto herein as a “barrier modulated cell” (BMC).

In some cases, each memory cell in a cross-point memory array includes areversible resistance-switching memory element in series with a steeringelement or an isolation element, such as a diode, to reduce leakagecurrents. In other cross-point memory arrays, the memory cells do notinclude an isolation element.

In an embodiment, a non-volatile storage system may include one or moretwo-dimensional arrays of non-volatile memory cells. The memory cellswithin a two-dimensional memory array may form a single layer of memorycells and may be selected via control lines (e.g., word lines and bitlines) in the X and Y directions. In another embodiment, a non-volatilestorage system may include one or more monolithic three-dimensionalmemory arrays in which two or more layers of memory cells may be formedabove a single substrate without any intervening substrates.

In some cases, a three-dimensional memory array may include one or morevertical columns of memory cells located above and orthogonal to asubstrate. In an example, a non-volatile storage system may include amemory array with vertical bit lines or bit lines that are arrangedorthogonal to a semiconductor substrate. The substrate may include asilicon substrate. The memory array may include rewriteable non-volatilememory cells, wherein each memory cell includes a reversibleresistance-switching memory element without an isolation element inseries with the reversible resistance-switching memory element (e.g., nodiode in series with the reversible resistance-switching memoryelement).

In some embodiments, a non-volatile storage system may include anon-volatile memory that is monolithically formed in one or morephysical levels of arrays of memory cells having an active area disposedabove a silicon substrate. The non-volatile storage system may alsoinclude circuitry associated with the operation of the memory cells(e.g., decoders, state machines, page registers, and/or controlcircuitry for controlling reading, programming and erasing of the memorycells). The circuitry associated with the operation of the memory cellsmay be located above the substrate or within the substrate.

In some embodiments, a non-volatile storage system may include amonolithic three-dimensional memory array. The monolithicthree-dimensional memory array may include one or more levels of memorycells. Each memory cell within a first level of the one or more levelsof memory cells may include an active area that is located above asubstrate (e.g., above a single-crystal substrate or a crystallinesilicon substrate). In one example, the active area may include asemiconductor junction (e.g., a P-N junction). The active area mayinclude a portion of a source or drain region of a transistor. Inanother example, the active area may include a channel region of atransistor.

FIG. 1A depicts one embodiment of a memory system 100 and a host 102.Memory system 100 may include a non-volatile storage system interfacingwith host 102 (e.g., a mobile computing device). In some cases, memorysystem 100 may be embedded within host 102. In other cases, memorysystem 100 may include a memory card. As depicted, memory system 100includes a memory chip controller 104 and a memory chip 106. Although asingle memory chip 106 is depicted, memory system 100 may include morethan one memory chip (e.g., four, eight or some other number of memorychips). Memory chip controller 104 may receive data and commands fromhost 102 and provide memory chip data to host 102.

Memory chip controller 104 may include one or more state machines, pageregisters, SRAM, and control circuitry for controlling the operation ofmemory chip 106. The one or more state machines, page registers, SRAM,and control circuitry for controlling the operation of memory chip 106may be referred to as managing or control circuits. The managing orcontrol circuits may facilitate one or more memory array operations,such as forming, erasing, programming, and reading operations.

In some embodiments, the managing or control circuits (or a portion ofthe managing or control circuits) for facilitating one or more memoryarray operations may be integrated within memory chip 106. Memory chipcontroller 104 and memory chip 106 may be arranged on a singleintegrated circuit. In other embodiments, memory chip controller 104 andmemory chip 106 may be arranged on different integrated circuits. Insome cases, memory chip controller 104 and memory chip 106 may beintegrated on a system board, logic board, or a PCB.

Memory chip 106 includes memory core control circuits 108 and a memorycore 110. Memory core control circuits 108 may include logic forcontrolling the selection of memory blocks (or arrays) within memorycore 110, controlling the generation of voltage references for biasing aparticular memory array into a read or write state, and generating rowand column addresses.

Memory core 110 may include one or more two-dimensional arrays of memorycells or one or more three-dimensional arrays of memory cells. In anembodiment, memory core control circuits 108 and memory core 110 arearranged on a single integrated circuit. In other embodiments, memorycore control circuits 108 (or a portion of memory core control circuits108) and memory core 110 may be arranged on different integratedcircuits.

A memory operation may be initiated when host 102 sends instructions tomemory chip controller 104 indicating that host 102 would like to readdata from memory system 100 or write data to memory system 100. In theevent of a write (or programming) operation, host 102 will send tomemory chip controller 104 both a write command and the data to bewritten. The data to be written may be buffered by memory chipcontroller 104 and error correcting code (ECC) data may be generatedcorresponding with the data to be written. The ECC data, which allowsdata errors that occur during transmission or storage to be detectedand/or corrected, may be written to memory core 110 or stored innon-volatile memory within memory chip controller 104. In an embodiment,the ECC data are generated and data errors are corrected by circuitrywithin memory chip controller 104.

Memory chip controller 104 controls operation of memory chip 106. In oneexample, before issuing a write operation to memory chip 106, memorychip controller 104 may check a status register to make sure that memorychip 106 is able to accept the data to be written. In another example,before issuing a read operation to memory chip 106, memory chipcontroller 104 may pre-read overhead information associated with thedata to be read. The overhead information may include ECC dataassociated with the data to be read or a redirection pointer to a newmemory location within memory chip 106 in which to read the datarequested. Once a read or write operation is initiated by memory chipcontroller 104, memory core control circuits 108 may generate theappropriate bias voltages for word lines and bit lines within memorycore 110, and generate the appropriate memory block, row, and columnaddresses.

In some embodiments, one or more managing or control circuits may beused for controlling the operation of a memory array. The one or moremanaging or control circuits may provide control signals to a memoryarray to perform an erase operation, a read operation, and/or a writeoperation on the memory array. In one example, the one or more managingor control circuits may include any one of or a combination of controlcircuitry, state machine, decoders, sense amplifiers, read/writecircuits, and/or controllers. The one or more managing circuits mayperform or facilitate one or more memory array operations includingerasing, programming, or reading operations. In one example, one or moremanaging circuits may include an on-chip memory controller fordetermining row and column address, word line and bit line addresses,memory array enable signals, and data latching signals.

FIG. 1B depicts one embodiment of memory core control circuits 108. Asdepicted, memory core control circuits 108 include address decoders 120,voltage generators for first control lines 122, voltage generators forsecond control lines 124 and signal generators for reference signals 126(described in more detail below). Control lines may include word lines,bit lines, or a combination of word lines and bit lines. First controllines may include first (e.g., selected) word lines and/or first (e.g.,selected) bit lines that are used to place memory cells into a first(e.g., selected) state. Second control lines may include second (e.g.,unselected) word lines and/or second (e.g., unselected) bit lines thatare used to place memory cells into a second (e.g., unselected) state.

Address decoders 120 may generate memory block addresses, as well as rowaddresses and column addresses for a particular memory block. Voltagegenerators (or voltage regulators) for first control lines 122 mayinclude one or more voltage generators for generating first (e.g.,selected) control line voltages. Voltage generators for second controllines 124 may include one or more voltage generators for generatingsecond (e.g., unselected) control line voltages. Signal generators forreference signals 126 may include one or more voltage and/or currentgenerators for generating reference voltage and/or current signals.

FIGS. 1C-1F depict one embodiment of a memory core organization thatincludes a memory core having multiple memory bays, and each memory bayhaving multiple memory blocks. Although a memory core organization isdisclosed where memory bays include memory blocks, and memory blocksinclude a group of memory cells, other organizations or groupings alsocan be used with the technology described herein.

FIG. 1C depicts an embodiment of memory core 110 of FIG. 1A. Asdepicted, memory core 110 includes memory bay 130 and memory bay 132. Insome embodiments, the number of memory bays per memory core can differfor different implementations. For example, a memory core may includeonly a single memory bay or multiple memory bays (e.g., 16 or othernumber of memory bays).

FIG. 1D depicts an embodiment of memory bay 130 in FIG. 1C. As depicted,memory bay 130 includes memory blocks 140-144 and read/write circuits146. In some embodiments, the number of memory blocks per memory bay maydiffer for different implementations. For example, a memory bay mayinclude one or more memory blocks (e.g., 32 or other number of memoryblocks per memory bay). Read/write circuits 146 include circuitry forreading and writing memory cells within memory blocks 140-144.

As depicted, read/write circuits 146 may be shared across multiplememory blocks within a memory bay. This allows chip area to be reducedbecause a single group of read/write circuits 146 may be used to supportmultiple memory blocks. However, in some embodiments, only a singlememory block may be electrically coupled to read/write circuits 146 at aparticular time to avoid signal conflicts.

In some embodiments, read/write circuits 146 may be used to write one ormore pages of data into memory blocks 140-144 (or into a subset of thememory blocks). The memory cells within memory blocks 140-144 may permitdirect over-writing of pages (i.e., data representing a page or aportion of a page may be written into memory blocks 140-144 withoutrequiring an erase or reset operation to be performed on the memorycells prior to writing the data).

In one example, memory system 100 of FIG. 1A may receive a write commandincluding a target address and a set of data to be written to the targetaddress. Memory system 100 may perform a read-before-write (RBW)operation to read the data currently stored at the target address and/orto acquire overhead information (e.g., ECC information) beforeperforming a write operation to write the set of data to the targetaddress.

In some cases, read/write circuits 146 may be used to program aparticular memory cell to be in one of three or more data/resistancestates (i.e., the particular memory cell may include a multi-levelmemory cell). In one example, read/write circuits 146 may apply a firstvoltage difference (e.g., 2V) across the particular memory cell toprogram the particular memory cell into a first state of the three ormore data/resistance states or a second voltage difference (e.g., 1V)across the particular memory cell that is less than the first voltagedifference to program the particular memory cell into a second state ofthe three or more data/resistance states.

Applying a smaller voltage difference across the particular memory cellmay cause the particular memory cell to be partially programmed orprogrammed at a slower rate than when applying a larger voltagedifference. In another example, read/write circuits 146 may apply afirst voltage difference across the particular memory cell for a firsttime period to program the particular memory cell into a first state ofthe three or more data/resistance states, and apply the first voltagedifference across the particular memory cell for a second time periodless than the first time period. One or more programming pulses followedby a memory cell verification phase may be used to program theparticular memory cell to be in the correct state.

FIG. 1E depicts an embodiment of memory block 140 in FIG. 1D. Asdepicted, memory block 140 includes a memory array 150, row decoder 152,and column decoder 154. Memory array 150 may include a contiguous groupof memory cells having contiguous word lines and bit lines. Memory array150 may include one or more layers of memory cells. Memory array 150 mayinclude a two-dimensional memory array or a three-dimensional memoryarray.

Row decoder 152 decodes a row address and selects a particular word linein memory array 150 when appropriate (e.g., when reading or writingmemory cells in memory array 150). Column decoder 154 decodes a columnaddress and selects one or more bit lines in memory array 150 to beelectrically coupled to read/write circuits, such as read/write circuits146 in FIG. 1D. In one embodiment, the number of word lines is 4K permemory layer, the number of bit lines is 1K per memory layer, and thenumber of memory layers is 4, providing a memory array 150 containing16M memory cells.

FIG. 1F depicts an embodiment of a memory bay 134. Memory bay 134 is analternative example implementation for memory bay 130 of FIG. 1D. Insome embodiments, row decoders, column decoders, and read/write circuitsmay be split or shared between memory arrays. As depicted, row decoder152 b is shared between memory arrays 150 a and 150 b because rowdecoder 152 b controls word lines in both memory arrays 150 a and 150 b(i.e., the word lines driven by row decoder 152 b are shared).

Row decoders 152 a and 152 b may be split such that even word lines inmemory array 150 a are driven by row decoder 152 a and odd word lines inmemory array 150 a are driven by row decoder 152 b. Row decoders 152 cand 152 b may be split such that even word lines in memory array 150 bare driven by row decoder 152 c and odd word lines in memory array 150 bare driven by row decoder 152 b.

Column decoders 154 a and 154 b may be split such that even bit lines inmemory array 150 a are controlled by column decoder 154 b and odd bitlines in memory array 150 a are driven by column decoder 154 a. Columndecoders 154 c and 154 d may be split such that even bit lines in memoryarray 150 b are controlled by column decoder 154 d and odd bit lines inmemory array 150 b are driven by column decoder 154 c.

The selected bit lines controlled by column decoder 154 a and columndecoder 154 c may be electrically coupled to read/write circuits 146 a.The selected bit lines controlled by column decoder 154 b and columndecoder 154 d may be electrically coupled to read/write circuits 146 b.Splitting the read/write circuits into read/write circuits 146 a and 146b when the column decoders are split may allow for a more efficientlayout of the memory bay.

FIG. 2A depicts one embodiment of a portion of a monolithicthree-dimensional memory array 200 that includes a first memory level210, and a second memory level 212 positioned above first memory level210. Memory array 200 is one example of an implementation for memoryarray 150 of FIG. 1E. Local bit lines LBL₁₁-LBL₃₃ are arranged in afirst direction (e.g., a vertical or z-direction) and word linesWL₁₀-WL₂₃ are arranged in a second direction (e.g., an x-direction)perpendicular to the first direction. This arrangement of vertical bitlines in a monolithic three-dimensional memory array is one embodimentof a vertical bit line memory array.

As depicted, disposed between the intersection of each local bit lineand each word line is a particular memory cell (e.g., memory cell M₁₁₁is disposed between local bit line LBL₁₁ and word line WL₁₀). Theparticular memory cell may include a floating gate memory element, acharge trap memory element (e.g., using a silicon nitride material), areversible resistance-switching memory element, or other similar device.The global bit lines GBL₁-GBL₃ are arranged in a third direction (e.g.,a y-direction) that is perpendicular to both the first direction and thesecond direction.

Each local bit line LBL₁₁-LBL₃₃ has an associated bit line selecttransistor Q₁₁-Q₃₃, respectively. Bit line select transistors Q₁₁-Q₃₃may be field effect transistors, such as shown, or may be any othertransistors. As depicted, bit line select transistors Q₁₁-Q₃₁ areassociated with local bit lines LBL₁₁-LBL₃₁, respectively, and may beused to connect local bit lines LBL₁₁-LBL₃₁ to global bit linesGBL₁-GBL₃, respectively, using row select line SG₁. In particular, eachof bit line select transistors Q₁₁-Q₃₁ has a first terminal (e.g., adrain/source terminal) coupled to a corresponding one of local bit linesLBL₁₁-LBL₃₁, respectively, a second terminal (e.g., a source/drainterminal) coupled to a corresponding one of global bit lines GBL₁-GBL₃,respectively, and a third terminal (e.g., a gate terminal) coupled torow select line SG₁.

Similarly, bit line select transistors Q₁₂-Q₃₂ are associated with localbit lines LBL₁₂-LBL₃₂, respectively, and may be used to connect localbit lines LBL₁₂-LBL₃₂ to global bit lines GBL₁-GBL₃, respectively, usingrow select line SG₂. In particular, each of bit line select transistorsQ₁₂-Q₃₂ has a first terminal (e.g., a drain/source terminal) coupled toa corresponding one of local bit lines LBL₁₂-LBL₃₂, respectively, asecond terminal (e.g., a source/drain terminal) coupled to acorresponding one of global bit lines GBL₁-GBL₃, respectively, and athird terminal (e.g., a gate terminal) coupled to row select line SG₂.

Likewise, bit line select transistors Q₁₃-Q₃₃ are associated with localbit lines LBL₁₃-LBL₃₃, respectively, and may be used to connect localbit lines LBL₁₃-LBL₃₃ to global bit lines GBL₁-GBL₃, respectively, usingrow select line SG₃. In particular, each of bit line select transistorsQ₁₃-Q₃₃ has a first terminal (e.g., a drain/source terminal) coupled toa corresponding one of local bit lines LBL₁₃-LBL₃₃, respectively, asecond terminal (e.g., a source/drain terminal) coupled to acorresponding one of global bit lines GBL₁-GBL₃, respectively, and athird terminal (e.g., a gate terminal) coupled to row select line SG₃.

Because a single bit line select transistor is associated with acorresponding local bit line, the voltage of a particular global bitline may be selectively applied to a corresponding local bit line.Therefore, when a first set of local bit lines (e.g., LBL₁₁-LBL₃₁) isbiased to global bit lines GBL₁-GBL₃, the other local bit lines (e.g.,LBL₁₂-LBL₃₂ and LBL₁₃-LBL₃₃) must either also be driven to the sameglobal bit lines GBL₁-GBL₃ or be floated.

In an embodiment, during a memory operation, all local bit lines withinthe memory array are first biased to an unselected bit line voltage byconnecting each of the global bit lines to one or more local bit lines.After the local bit lines are biased to the unselected bit line voltage,then only a first set of local bit lines LBL₁₁-LBL₃₁ are biased to oneor more selected bit line voltages via the global bit lines GBL₁-GBL₃,while the other local bit lines (e.g., LBL₁₂-LBL₃₂ and LBL₁₃-LBL₃₃) arefloated. The one or more selected bit line voltages may correspond with,for example, one or more read voltages during a read operation or one ormore programming voltages during a programming operation.

In an embodiment, a vertical bit line memory array, such as memory array200, includes a greater number of memory cells along the word lines ascompared with the number of memory cells along the vertical bit lines(e.g., the number of memory cells along a word line may be more than 10times the number of memory cells along a bit line). In one example, thenumber of memory cells along each bit line may be 16 or 32, whereas thenumber of memory cells along each word line may be 2048 or more than4096. Other numbers of memory cells along each bit line and along eachword line may be used.

In an embodiment of a read operation, the data stored in a selectedmemory cell (e.g., memory cell M₁₁₁) may be read by biasing the wordline connected to the selected memory cell (e.g., selected word lineWL₁₀) to a selected word line voltage in read mode (e.g., 0V). The localbit line (e.g., LBL₁₁) coupled to the selected memory cell (M₁₁₁) isbiased to a selected bit line voltage in read mode (e.g., 1 V) via theassociated bit line select transistor (e.g., Q₁₁) coupled to theselected local bit line (LBL₁₁), and the global bit line (e.g., GBL₁)coupled to the bit line select transistor (Q₁₁). A sense amplifier maythen be coupled to the selected local bit line (LBL₁₁) to determine aread current I_(READ) of the selected memory cell (M₁₁₁). The readcurrent I_(READ) is conducted by the bit line select transistor Q₁₁, andmay be between about 100 nA and about 500 nA, although other readcurrents may be used.

In an embodiment of a write operation, data may be written to a selectedmemory cell (e.g., memory cell M₂₂₁) by biasing the word line connectedto the selected memory cell (e.g., WL₂₀) to a selected word line voltagein write mode (e.g., 5V). The local bit line (e.g., LBL₂₁) coupled tothe selected memory cell (M₂₂₁) is biased to a selected bit line voltagein write mode (e.g., 0 V) via the associated bit line select transistor(e.g., Q₂₁) coupled to the selected local bit line (LBL₂₁), and theglobal bit line (e.g., GBL₂) coupled to the bit line select transistor(Q₂₁). During a write operation, a programming current I_(PGRM) isconducted by the associated bit line select transistor Q₂₁, and may bebetween about 3 uA and about 6 uA, although other programming currentsmay be used.

During the write operation described above, the word line (e.g., WL₂₀)connected to the selected memory cell (M₂₂₁) may be referred to as a“selected word line,” and the local bit line (e.g., LBL₂₁) coupled tothe selected memory cell (M₂₂₁) be referred to as the “selected localbit line.” All other word lines coupled to unselected memory cells maybe referred to as “unselected word lines,” and all other local bit linescoupled to unselected memory cells may be referred to as “unselectedlocal bit lines.” For example, if memory cell M₂₂₁ is the only selectedmemory cell in memory array 200, word lines WL₁₀-WL₁₃ and WL₂₁-WL₂₃ areunselected word lines, and local bit lines LBL₁₁, LBL₃₁, LBL₁₂-LBL₃₂,and LBL₁₃-LBL₃₃ are unselected local bit lines.

FIG. 2B depicts an embodiment of a portion of a monolithicthree-dimensional memory array 202 that includes vertical strips of anon-volatile memory material. The portion of monolithicthree-dimensional memory array 202 depicted in FIG. 2B may include animplementation for a portion of the monolithic three-dimensional memoryarray 200 depicted in FIG. 2A.

Monolithic three-dimensional memory array 202 includes word lines WL₁₀,WL₁₁, WL₁₂, . . . , WL₄₂ that are formed in a first direction (e.g., anx-direction), vertical bit lines LBL₁₁, LBL₁₂, LBL13, . . . , LBL₂₃ thatare formed in a second direction perpendicular to the first direction(e.g., a z-direction), and vertical strips of non-volatile memorymaterial 214 formed in the second direction (e.g., the z-direction). Aspacer 216 made of a dielectric material (e.g., silicon dioxide, siliconnitride, or other dielectric material) is disposed between adjacent wordlines WL₁₀, WL₁₁, WL₄₂, . . . , WL₄₂.

Each vertical strip of non-volatile memory material 214 may include, forexample, a vertical oxide material, a vertical reversibleresistance-switching memory material (e.g., one or more metal oxidelayers such as nickel oxide, hafnium oxide, or other similar metal oxidematerials, a phase change material, a barrier modulated switchingstructure or other similar reversible resistance-switching memorymaterial), a ferroelectric material, or other non-volatile memorymaterial.

Each vertical strip of non-volatile memory material 214 may include asingle material layer or multiple material layers. In an embodiment,each vertical strip of non-volatile memory material 214 includes avertical barrier modulated switching structure. Example barriermodulated switching structures include a semiconductor material layeradjacent a conductive oxide material layer (e.g., an amorphous siliconlayer adjacent a crystalline titanium oxide layer). Other examplebarrier modulated switching structures include a thin (e.g., less thanabout 2 nm) barrier oxide material disposed between the semiconductormaterial layer and the conductive oxide material layer (e.g., analuminum oxide layer disposed between an amorphous silicon layer and acrystalline titanium oxide layer). Such multi-layer embodiments may beused to form BMC memory elements.

In an embodiment, each vertical strip of non-volatile memory material214 may include a single continuous layer of material that may be usedby a plurality of memory cells or devices.

In an embodiment, portions of the vertical strip of the non-volatilememory material 214 b may include a part of a first memory cellassociated with the cross section between WL₁₂ and LBL₁₃ and a part of asecond memory cell associated with the cross section between WL₂₂ andLBL₁₃. In some cases, a vertical bit line, such as LBL₁₃, may include avertical structure (e.g., a rectangular prism, a cylinder, or a pillar)and the non-volatile material may completely or partially surround thevertical structure (e.g., a conformal layer of phase change materialsurrounding the sides of the vertical structure).

As depicted, each of the vertical bit lines LBL₁₁, LBL₁₂, LBL13, . . . ,LBL₂₃ may be connected to one of a set of global bit lines via anassociated vertically-oriented bit line select transistor (e.g., Q₁₁,Q₁₂, Q₁₃, Q₂₃). Each vertically-oriented bit line select transistor mayinclude a MOS device (e.g., an NMOS device) or a vertical thin-filmtransistor (TFT).

In an embodiment, each vertically-oriented bit line select transistor isa vertically-oriented pillar-shaped TFT coupled between an associatedlocal bit line pillar and a global bit line. In an embodiment, thevertically-oriented bit line select transistors are formed in a pillarselect layer formed above a CMOS substrate, and a memory layer thatincludes multiple layers of word lines and memory elements is formedabove the pillar select layer.

FIGS. 3A-3E depict various views of an embodiment of a portion of amonolithic three-dimensional memory array 300 that includes verticalstrips of a non-volatile memory material. The physical structuredepicted in FIGS. 3A-3E may include one implementation for a portion ofthe monolithic three-dimensional memory array depicted in FIG. 2B.

Monolithic three-dimensional memory array 300 includes vertical bitlines LBL₁₁-LBL₃₃ arranged in a first direction (e.g., a z-direction),word lines WL₁₀, WL₁₁, . . . , WL₅₃ arranged in a second direction(e.g., an x-direction) perpendicular to the first direction, and rowselect lines SG₁, SG₂, SG₃ arranged in the second direction, and globalbit lines GBL₁, GBL₂, GBL₃ arranged in a third direction (e.g., ay-direction) perpendicular to the first and second directions.

Vertical bit lines LBL₁₁-LBL₃₃ are disposed above global bit lines GBL₁,GBL₂, GBL₃, which each have a long axis in the second (e.g.,x-direction). Person of ordinary skill in the art will understand thatmonolithic three-dimensional memory arrays, such as monolithicthree-dimensional memory array 300 may include more or fewer than twentyword lines, three row select lines, three global bit lines, and ninevertical bit lines.

In an embodiment, global bit lines GBL₁, GBL₂, GBL₃ are disposed above asubstrate 302, such as a silicon, germanium, silicon-germanium, undoped,doped, bulk, silicon-on-insulator (“SOI”) or other substrate with orwithout additional circuitry. In an embodiment, an isolation layer 304,such as a layer of silicon dioxide, silicon nitride, silicon oxynitrideor any other suitable insulating layer, is formed above substrate 302.

In an embodiment, a first dielectric material layer 308 (e.g., silicondioxide) and a second dielectric material layer 310 (e.g., silicondioxide) are formed above isolation layer 304. Global bit lines GBL₁,GBL₂, GBL₃ are disposed above isolation layer 304 and are separated fromone another by first dielectric material layer 308.

Vertically-oriented bit line select transistors Q₁₁-Q₃₃ are disposedabove global bit lines GBL₁, GBL₂, GBL₃ and are separated from oneanother by second dielectric material layer 310. Vertically-oriented bitline select transistors Q₁₁-Q₁₃ are disposed above and electricallycoupled to global bit line GBL₁, vertically-oriented bit line selecttransistors Q₂₁-Q₂₃ are disposed above and electrically coupled toglobal bit line GBL₂, and vertically-oriented bit line selecttransistors Q₃₁-Q₃₃ are disposed above and electrically coupled toglobal bit line GBL₃.

Vertically-oriented bit line select transistors Q₁₁-Q₃₃ may be fieldeffect transistors, although other transistors types may be used. In anembodiment, each of vertically-oriented bit line select transistorsQ₃₁-Q₃₃ has a height between about 1500 angstroms and about 5000angstroms. Other height values may be used.

Each of vertically-oriented bit line select transistors Q₁₁-Q₃₃ has afirst terminal 312 a (e.g., a drain/source terminal), a second terminal312 b (e.g., a source/drain terminal), a first control terminal 312 c 1(e.g., a first gate terminal) and a second control terminal 312 c 2(e.g., a second gate terminal). First gate terminal 312 c 1 and secondgate terminal 312 c 2 may be disposed on opposite sides of thevertically-oriented bit line select transistor. A gate dielectricmaterial 314 (e.g., SiO₂) is disposed between first gate terminal 312 c1 and first terminal 312 a and second terminal 312 b, and also isdisposed between second gate terminal 312 c 2 and first terminal 312 aand second terminal 312 b.

First gate terminal 312 c 1 may be used to selectively induce a firstconductive channel between first terminal 312 a and second terminal 312b of the transistor, and second gate terminal 312 c 2 may be used toselectively induce a second conductive channel between first terminal312 a and second terminal 312 b of the transistor. In an embodiment,first gate terminal 312 c 1 and second gate terminal 312 c 2 are coupledtogether to form a single control terminal 312 c that may be used tocollectively turn ON and OFF the vertically-oriented bit line selecttransistor.

Row select lines SG₁, SG₂, SG₃ are disposed above global bit lines GBL₁,GBL₂ and GBL₃, and form gate terminals 312 c of vertically-oriented bitline select transistors Q₁₁-Q₃₃. In particular, row select line SG₁forms the gate terminals of vertically-oriented bit line selecttransistors Q₁₁, Q₂₁ and Q₃₁, row select line SG₂ forms the gateterminals of vertically-oriented bit line select transistors Q₁₂, Q₂₂and Q₃₂, and row select line SG₃ forms the gate terminals ofvertically-oriented bit line select transistors Q₁₃, Q₂₃ and Q₃₃.

A first etch stop layer 316 (e.g., aluminum oxide) is disposed abovesecond dielectric material layer 310. A stack of word lines WL₁₀, WL₁₁,. . . , WL₅₃ is disposed above first etch stop layer 316, with a thirddielectric material layer 318 (e.g., silicon dioxide) separatingadjacent word lines. A second etch stop layer 320 (e.g., polysilicon)may be formed above the stack of word lines WL₁₀, WL₁₁, . . . , WL₅₃.

In an embodiment, vertical strips of a non-volatile memory material 214are disposed adjacent word lines WL₁₀, WL₁₁, . . . , WL₅₃. Verticalstrips of non-volatile memory material 214 may be formed in the firstdirection (e.g., the z-direction). A vertical strip of non-volatilememory material 214 may include, for example, a vertical oxide layer, avertical reversible resistance-switching material (e.g., one or moremetal oxide layers such as nickel oxide, hafnium oxide, or other similarmetal oxide materials, a phase change material, a barrier modulatedswitching structure or other similar reversible resistance-switchingmemory material), a ferroelectric material, or other non-volatile memorymaterial.

A vertical strip of non-volatile memory material 214 may include asingle continuous layer of material that may be used by a plurality ofmemory cells or devices. For simplicity, vertical strip of non-volatilememory material 214 will be referred to in the remaining discussion asreversible resistance-switching memory material 214.

Each vertical strip of non-volatile memory material 214 may include asingle material layer or multiple material layers. In an embodiment,each vertical strip of non-volatile memory material 214 includes avertical barrier modulated switching structure that includes asemiconductor material layer adjacent a conductive oxide material layer.In other embodiments, each vertical strip of non-volatile memorymaterial 214 includes a vertical barrier modulated switching structurethat includes a thin (e.g., less than about 2 nm) barrier oxide materialdisposed between a semiconductor material layer and a conductive oxidematerial layer.

In an embodiment, each vertical strip of non-volatile memory material214 includes a barrier modulated switching structure that includes asemiconductor material layer 322 and a conductive oxide material layer324. In embodiments, semiconductor material layer 322 includes one ormore of amorphous silicon, amorphous tantalum nitride, amorphoustantalum silicon nitride, or other similar semiconductor material, andconductive oxide material layer 324 includes one or more of crystallinetitanium oxide, crystalline zinc oxide, crystalline tungsten oxide,crystalline praseodymium calcium manganese oxide, or other similarconductive oxide material. Other semiconductor materials and/orconductive oxide materials may be used. As described above, a BMC memorycell includes a barrier modulated switching structure.

Vertical bit lines LBL₁₁-LBL₃₃ are disposed adjacent reversibleresistance-switching memory material 214, are formed of a conductivematerial (e.g., titanium nitride). In an embodiment, each of verticalbit lines LBL₁₁-LBL₃₃ includes an adhesion material layer (not shown)disposed adjacent reversible resistance-switching memory material 214.Vertical bit lines LBL₁₁-LBL₃₃ are separated from one another by afourth dielectric material layer 326 (e.g., silicon dioxide). In someembodiments, each of a vertical bit lines LBL₁₁-LBL₃₃ includes avertical structure (e.g., a rectangular prism, a cylinder, or a pillar),and the vertical strip of reversible resistance-switching memorymaterial 214 may completely or partially surround the vertical structure(e.g., a conformal layer of reversible resistance-switching materialsurrounding the sides of the vertical structure).

A memory cell is disposed between the intersection of each vertical bitline and each word line. For example, a memory cell M₁₁₁ is disposedbetween vertical bit line LBL₁₁ and word line WL₁₀, a memory cell M₁₁₆is disposed between vertical bit line LBL₁₃ and word line WL₁₃, a memorycell M₅₁₁ is disposed between vertical bit line LBL₁₁ and word lineWL₅₀, a memory cell M₅₃₆ is disposed between vertical bit line LBL₃₃ andword line WL₅₀, and so on. In an embodiment, monolithicthree-dimensional memory array 300 includes ninety memory cells M₁₁₁,M₁₁₂, . . . , M₅₃₆. Persons of ordinary skill in the art will understandthat monolithic three-dimensional memory arrays may include more orfewer than ninety memory cells.

In an example, portions of the vertical strip of reversibleresistance-switching material 214 may include a part of memory cell M₁₁₁associated with the cross section between word line WL₁₀ and LBL₁₁, anda part of memory cell M₂₁₁ associated with the cross section betweenword line WL₂₀ and LBL₁₁, and so on.

Each of memory cells M₁₁₁, M₁₁₂, . . . , M₅₃₆ may include a floatinggate device, a charge trap device (e.g., using a silicon nitridematerial), a resistive change memory device, or other type of memorydevice. Vertically-oriented bit line select transistors Q₁₁-Q₃₃ may beused to select a corresponding one of vertical bit lines LBL₁₁-LBL₃₃.Vertically-oriented bit line select transistors Q₁₁-Q₃₃ may be fieldeffect transistors, although other transistors types may be used.

Thus, the first gate terminal and the second gate terminal of each ofvertically-oriented bit line select transistors Q₁₁-Q₃₃ may be used toturn ON and OFF vertically-oriented bit line select transistors Q₁₁-Q₃.Without wanting to be bound by any particular theory, for each ofvertically-oriented bit line select transistors Q₁₁-Q₃₃, it is believedthat the current drive capability of the transistor may be increased byusing both the first gate terminal and the second gate terminal to turnON the transistor. For simplicity, the first and second gate terminal ofeach of select transistors Q₁₁-Q₃₃ will be referred to as a single gateterminal.

Vertically-oriented bit line select transistors Q₁₁, Q₁₂, Q₁₃ are usedto selectively connect/disconnect vertical bit lines LBL₁₁, LBL₁₂, andLBL₁₃ to/from global bit line GBL₁ using row select lines SG₁, SG₂, SG₃,respectively. In particular, each of vertically-oriented bit line selecttransistors Q₁₁, Q₁₂, Q₁₃ has a first terminal (e.g., a drain/sourceterminal) coupled to a corresponding one of vertical bit lines LBL₁₁,LBL₁₂, and LBL₁₃, respectively, a second terminal (e.g., a source/drainterminal) coupled to global bit line GBL₁, and a control terminal (e.g.,a gate terminal) coupled to row select line SG₁, SG₂, SG₃, respectively.

Row select lines SG₁, SG₂, SG₃ are used to turn ON/OFFvertically-oriented bit line select transistors Q₁₁, Q₁₂, Q₁₃,respectively, to connect/disconnect vertical bit lines LBL₁₁, LBL₁₂, andLBL₁₃, respectively, to/from global bit line GBL₁.

Likewise, vertically-oriented bit line select transistors Q₁₁, Q₂₁, . .. , Q₃₃ are used to selectively connect/disconnect vertical bit linesLBL₁₁, LBL₂₁, and LBL₃₁, respectively, to global bit lines GBL₁, GBL₂,GBL₃, respectively, using row select line SG₁. In particular, each ofvertically-oriented bit line select transistors Q₁₁, Q₂₁, Q₃₁ has afirst terminal (e.g., a drain/source terminal) coupled to acorresponding one of vertical bit lines LBL₁₁, LBL₂₁, and LBL₃₁,respectively, a second terminal (e.g., a source/drain terminal) coupledto a corresponding one of global bit lines GBL₁, GBL₂, GBL₃,respectively, and a control terminal (e.g., a gate terminal) coupled torow select line SG₁. Row select line SG₁ is used to turn ON/OFFvertically-oriented bit line select transistors Q₁₁, Q₂₁, Q₃₁ toconnect/disconnect vertical bit lines LBL₁₁, LBL₂₁, and LBL₃₁,respectively, to/from global bit lines GBL₁, GBL₂, GBL₃, respectively.

Similarly, vertically-oriented bit line select transistors Q₁₃, Q₂₃, Q₃₃are used to selectively connect/disconnect vertical bit lines LBL₁₃,LBL₂₃, and LBL₃₃, respectively to/from global bit lines GBL₁, GBL₂,GBL₃, respectively, using row select line SG₃. In particular, each ofvertically-oriented bit line select transistors Q₁₃, Q₂₃, Q₃₃ has afirst terminal (e.g., a drain/source terminal) coupled to acorresponding one of vertical bit lines LBL₁₃, LBL₂₃, and LBL₃₃,respectively, a second terminal (e.g., a source/drain terminal) coupledto a corresponding one of global bit lines GBL₁, GBL₂, GBL₃,respectively, and a control terminal (e.g., a gate terminal) coupled torow select line SG₃. Row select line SG₃ is used to turn ON/OFFvertically-oriented bit line select transistors Q₁₃, Q₂₃, Q₃₃ toconnect/disconnect vertical bit lines LBL₁₃, LBL₂₃, and LBL₃₃,respectively, to/from global bit lines GBL₁, GBL₂, GBL₃, respectively.

One previously known monolithic three-dimensional memory array, such asmonolithic three-dimensional array 202 of FIG. 2B, includes titaniumnitride word lines and titanium nitride bit lines. BMC memory cellsformed using BMC memory elements disposed between titanium nitride wordlines and titanium nitride bit lines have been shown to require highoperation bias voltages for SET and RESET operations (e.g., SET >5V andRESET >6V). For many applications, such high operating bias voltages aretoo prohibitive.

Technology is described for forming BMC memory cells that include wordlines made from a first conductive material having a first workfunction, and bit lines made from a second conductive material having asecond work function, with the first work function greater than thesecond work function. In an embodiment, the first work function isgreater than the second work function by at least about 200 mV.

In an embodiment, technology is described for forming BMC memory cellsthat include tungsten or tungsten nitride word lines and titaniumnitride bit lines. Alternatively, word lines may be fabricated fromcobalt, ruthenium, or other similar material, and bit lines may befabricated from tantalum nitride, tantalum carbide, titanium carbide, orother similar material. Without wanting to be bound by any particulartheory, it is believed that by using word lines fabricated from aconductive material having a higher work function than that of the bitlines, reduced operating bias voltages may be used for SET and RESEToperations.

In particular, referring now to FIGS. 4A1-4J2, an example method offorming a monolithic three-dimensional memory array, such as monolithicthree-dimensional array 300 of FIGS. 3A-3E, is described.

With reference to FIGS. 4A1-4A3, substrate 302 is shown as havingalready undergone several processing steps. Substrate 302 may be anysuitable substrate such as a silicon, germanium, silicon-germanium,undoped, doped, bulk, silicon-on-insulator (“SOI”) or other substratewith or without additional circuitry. For example, substrate 302 mayinclude one or more n-well or p-well regions (not shown). Isolationlayer 304 is formed above substrate 302. In some embodiments, isolationlayer 304 may be a layer of silicon dioxide, silicon nitride, siliconoxynitride or any other suitable insulating layer.

Following formation of isolation layer 304, a conductive material layer306 is deposited over isolation layer 304. Conductive material layer 306may include any suitable conductive material such as tungsten or anotherappropriate metal, heavily doped semiconductor material, a conductivesilicide, a conductive silicide-germanide, a conductive germanide, orthe like deposited by any suitable method (e.g., CVD, PVD, etc.). In atleast one embodiment, conductive material layer 306 may comprise betweenabout 200 and about 2500 angstroms of tungsten. Other conductivematerial layers and/or thicknesses may be used. In some embodiments, anadhesion layer (not shown), such as titanium nitride or other similaradhesion layer material, may be disposed between isolation layer 304 andconductive material layer 306, and/or between conductive material layer306 and subsequent vertically-oriented bit line select transistorslayers.

Persons of ordinary skill in the art will understand that adhesionlayers may be formed by PVD or another method on conductive materiallayers. For example, adhesion layers may be between about 20 and about500 angstroms, and in some embodiments about 100 angstroms, of titaniumnitride or another suitable adhesion layer such as tantalum nitride,tungsten nitride, tungsten, molybdenum, combinations of one or moreadhesion layers, or the like. Other adhesion layer materials and/orthicknesses may be employed.

Following formation of conductive material layer 306, conductivematerial layer 306 is patterned and etched. For example, conductivematerial layer 306 may be patterned and etched using conventionallithography techniques, with a soft or hard mask, and wet or dry etchprocessing. In at least one embodiment, conductive material layer 306 ispatterned and etched to form global bit lines GBL₁, GBL₂, GBL₃. Examplewidths for global bit lines GBL₁, GBL₂, GBL₃ and/or spacings betweenglobal bit lines GBL₁, GBL₂, GBL₃ range between about 200 angstroms andabout 1000 angstroms, although other conductor widths and/or spacingsmay be used.

After global bit lines GBL₁, GBL₂, GBL₃ have been formed, a firstdielectric material layer 308 is formed over substrate 302 to fill thevoids between global bit lines GBL₁, GBL₂, GBL₃. For example,approximately 3000-7000 angstroms of silicon dioxide may be deposited onthe substrate 302 and planarized using chemical mechanical polishing oran etchback process to form a planar surface 400. Other dielectricmaterials such as silicon nitride, silicon oxynitride, low Kdielectrics, etc., and/or other dielectric material layer thicknessesmay be used. Example low K dielectrics include carbon doped oxides,silicon carbon layers, or the like.

In other embodiments, global bit lines GBL₁, GBL₂, GBL₃ may be formedusing a damascene process in which first dielectric material layer 308is formed, patterned and etched to create openings or voids for globalbit lines GBL₁, GBL₂, GBL₃. The openings or voids then may be filledwith conductive layer 306 (and/or a conductive seed, conductive filland/or barrier layer if needed). Conductive material layer 306 then maybe planarized to form planar surface 400.

Following planarization, the semiconductor material used to formvertically-oriented bit line select transistors Q₁₁-Q₃₃ is formed overplanar surface 400 of substrate 302. In some embodiments, eachvertically-oriented bit line select transistor is formed from apolycrystalline semiconductor material such as polysilicon, an epitaxialgrowth silicon, a polycrystalline silicon-germanium alloy, polygermaniumor any other suitable material. Alternatively, vertically-oriented bitline select transistors Q₁₁-Q₃₃ may be formed from a wide band-gapsemiconductor material, such as ZnO, InGaZnO, or SiC, which may providea high breakdown voltage, and typically may be used to providejunctionless FETs. Persons of ordinary skill in the art will understandthat other materials may be used.

In some embodiments, each vertically-oriented bit line select transistorQ₁₁-Q₃₃ may include a first region (e.g., p+ polysilicon), a secondregion (e.g., intrinsic polysilicon) and a third region (e.g., p+polysilicon) to form drain/source, body, and source/drain regions,respectively, of a vertical FET. For example, a heavily doped p+polysilicon layer 402 may be deposited on planar surface 400. P+ siliconmay be either deposited and doped by ion implantation or may be doped insitu during deposition to form p+ polysilicon layer 402.

For example, an intrinsic silicon layer may be deposited on planarsurface 400, and a blanket p-type implant may be employed to implantboron a predetermined depth within the intrinsic silicon layer. Exampleimplantable molecular ions include BF₂, BF₃, B and the like. In someembodiments, an implant dose of about 1-10×10¹³ ions/cm² may beemployed. Other implant species and/or doses may be used. Further, insome embodiments, a diffusion process may be employed. In an embodiment,the resultant p+ polysilicon layer 402 has a thickness of from about 50angstroms to about 300 angstroms, although other layer thicknessess maybe used.

Following formation of p+ polysilicon layer 402, an intrinsic (undoped)or lightly doped polysilicon layer 404 is deposited on p+ polysiliconlayer 402. In some embodiments, intrinsic layer 404 is in an amorphousstate as deposited. In other embodiments, intrinsic layer 404 is in apolycrystalline state as deposited. CVD or another suitable process maybe employed to deposit intrinsic layer 404. In an embodiment, intrinsiclayer 404 has a thickness between about 1000 angstroms to about 3000angstroms, although other layer thicknesses may be used.

After deposition of intrinsic layer 404, a p+ polysilicon layer 406 maybe formed over intrinsic layer 404. P-type silicon may be eitherdeposited and doped by ion implantation or may be doped in situ duringdeposition to form a p+ polysilicon layer 406.

For example, an intrinsic silicon layer may be deposited on intrinsiclayer 404, and a blanket p-type implant may be employed to implant borona predetermined depth within the intrinsic silicon layer. Exampleimplantable molecular ions include BF₂, BF₃, B and the like. In someembodiments, an implant dose of about 1-10×10¹³ ions/cm² may beemployed. Other implant species and/or doses may be used. Further, insome embodiments, a diffusion process may be employed. In an embodiment,the resultant p+ polysilicon layer 406 has a thickness of from about 50angstroms to about 300 angstroms, although other p-type silicon layersizes may be used.

Following formation of p+ polysilicon layer 406, silicon layers 402, 404and 406 are patterned and etched to form vertical transistor pillars.For example, silicon layers 402, 404 and 406 may be patterned and etchedusing conventional lithography techniques, with wet or dry etchprocessing. In an embodiment, silicon layers 402, 404 and 406 arepatterned and etched to form vertical transistor pillars disposed aboveglobal bit lines GBL₁, GBL₂, GBL₃. The vertical transistor pillars willbe used to form vertically-oriented bit line select transistors Q₁₁-Q₃₃.

Silicon layers 402, 404 and 406 may be patterned and etched in a singlepattern/etch procedure or using separate pattern/etch steps. Anysuitable masking and etching process may be used to form verticaltransistor pillars. For example, silicon layers may be patterned withabout 0.1 to about 1.5 micron of photoresist (“PR”) using standardphotolithographic techniques. Thinner PR layers may be used with smallercritical dimensions and technology nodes. In some embodiments, an oxidehard mask may be used below the PR layer to improve pattern transfer andprotect underlying layers during etching.

In some embodiments, after etching, the vertical transistor pillars maybe cleaned using a dilute hydrofluoric/sulfuric acid clean. Suchcleaning may be performed in any suitable cleaning tool, such as aRaider tool, available from Semitool of Kalispell, Mont. Examplepost-etch cleaning may include using ultra-dilute sulfuric acid (e.g.,about 1.5 1.8 wt %) for about 60 seconds and/or ultra-dilutehydrofluoric (“HF”) acid (e.g., about 0.4-0.6 wt %) for 60 seconds.Megasonics may or may not be used. Other clean chemistries, times and/ortechniques may be employed.

A gate dielectric material layer 314 is deposited conformally oversubstrate 302, and forms on sidewalls of the vertical transistorpillars. For example, between about 30 angstroms to about 100 angstromsof silicon dioxide may be deposited. Other dielectric materials such assilicon nitride, silicon oxynitride, low K dielectrics, etc., and/orother dielectric material layer thicknesses may be used.

Gate electrode material is deposited over the vertical transistorpillars and gate dielectric material layer 314 to fill the voids betweenthe vertical transistor pillar. For example, approximately 10 nm toabout 20 nm of titanium nitride or other similar metal, a highly-dopedsemiconductor, such as n+ polysilicon, p+ polysilicon, or other similarconductive material may be deposited. The as-deposited gate electrodematerial is subsequently etched back to form row select lines SG₁, SG₂,SG₃.

A second dielectric material layer 310 is deposited over substrate 302.For example, approximately 5000 to about 8000 angstroms of silicondioxide may be deposited and planarized using chemical mechanicalpolishing or an etch-back process to form planar top surface 408,resulting in the structure shown in FIGS. 4A1-4A3. Other dielectricmaterials and/or thicknesses may be used.

Planar surface 408 includes exposed top surfaces of vertically-orientedbit line select transistors Q₁₁-Q₃₃ and gate dielectric material layer314 separated by second dielectric material layer 310. Other dielectricmaterials such as silicon nitride, silicon oxynitride, low Kdielectrics, etc., and/or other dielectric material layer thicknessesmay be used. Example low K dielectrics include carbon doped oxides,silicon carbon layers, or the like.

A first etch stop layer 316 (e.g., aluminum oxide) is formed over planartop surface 408. A stack of alternating layers of third dielectricmaterial layer 318 and a sacrificial material layer 410 are formed overplanar top surface 408. Third dielectric material layers 318 may besilicon dioxide or other dielectric material. Sacrificial materiallayers 410 may include any suitable sacrificial material layers formedby any suitable method (e.g., CVD, PVD, etc.). Sacrificial materiallayers 410 each may be a nitride material, such as silicon nitride, asilicate glass, such as borophosphosilicate glass, a semiconductormaterial, such as amorphous silicon or polysilicon, or anothersemiconductor material, such as a group IV semiconductor, includingsilicon-germanium and germanium, or other sacrificial material.

In an embodiment, each third dielectric material layer 318 may bebetween about 50 angstroms and about 250 angstroms of SiO₂, and eachsacrificial material layer 410 may be between about 50 angstroms andabout 300 angstroms of silicon nitride. Other dielectric materialsand/or thicknesses and other sacrificial materials and/or thicknessesmay be used. In an embodiment, five sacrificial material layers 410 areformed over substrate 302. More or fewer than five sacrificial materiallayers 410 may be used.

Next, a second etch stop layer 320 is formed over substrate 302,resulting in the structure shown in FIGS. 4B1-4B2. Second etch stoplayer 320 may include any suitable etch stop layer formed by anysuitable method (e.g., CVD, PVD, etc.). In an embodiment, second etchstop layer 320 may be between about 50 angstroms and about 500 angstromsof polysilicon. Other etch stop layer materials and/or thicknesses maybe used.

Next, second etch stop layer 320, third dielectric material layers 318,and sacrificial material layers 410 are patterned and etched to formrows 412, with voids 414 separating rows 412, resulting in the structureshown in FIGS. 4C1-4C3. Each of rows 412 may be between about 200angstroms and about 1000 angstroms wide, although other widths may beused. Voids 414 may be between about 100 angstroms and about 800angstroms wide, although other widths may be used.

A fourth dielectric material 326 is deposited over substrate 302,filling voids 414 between rows 412. For example, approximately 3000-7000angstroms of silicon dioxide may be deposited on the substrate 302 andplanarized using chemical mechanical polishing or an etchback process toform a planar surface 416, resulting in the structure shown in FIGS.4D1-4D3. Other dielectric materials such as silicon nitride, siliconoxynitride, low K dielectrics, etc., and/or other dielectric materiallayer thicknesses may be used. Example low K dielectrics include carbondoped oxides, silicon carbon layers, or the like.

Next, fourth dielectric material 326 is patterned and etched to firstetch stop layer 316 to form holes 418 disposed above vertically-orientedbit line select transistors Q₁₁-Q₃₃, resulting in the structure shown inFIGS. 4E1-4E4. Although holes 418 are shown having a rectangular shape,other shapes may be used. In an embodiment, holes 418 may have a widthand a length of between about 200 angstroms and about 1000 angstroms.Other widths may be used.

An etch is used to remove sacrificial material layers 410 via holes 418to form cavities 420, resulting in the structure shown in FIGS. 4F1-4F4.In an embodiment, a wet etch may be used to remove sacrificial materiallayers 410. In other embodiments, wet and/or dry etch chemistries may beused to remove sacrificial material layers 410.

Next, a first conductive material layer having a first work function φ1is deposited over substrate 302 to fill cavities 420 via holes 418. Inan embodiment, cavities 420 may be lined with barrier and seedinglayers, and first conductive material layers, such as tungsten, tungstennitride, cobalt, ruthenium, or other similar material are formed on thebarrier and seeding layers. The barrier and seeding layers andconductive material layers form word lines WL₁₀, WL₁₁, . . . , WL₅₃,resulting in the structure shown in FIGS. 4G1-4G4.

A nonvolatile memory material layer 214 is deposited conformally oversubstrate 302. Non-volatile memory material 214 may include, forexample, an oxide layer, a reversible resistance-switching material(e.g., one or more metal oxide layers such as nickel oxide, hafniumoxide, or other similar metal oxide materials, a phase change material,a barrier modulated switching structure or other similar reversibleresistance-switching memory material), a ferroelectric material, orother nonvolatile memory material.

In an embodiment, each vertical strip of non-volatile memory material214 includes a barrier modulated switching structure that includes asemiconductor material layer 322 and a conductive oxide material layer324. In embodiments, semiconductor material layer 322 includes one ormore of amorphous silicon, amorphous tantalum nitride, amorphoustantalum silicon nitride, or other similar semiconductor material, andconductive oxide material layer 324 includes one or more of crystallinetitanium oxide, crystalline zinc oxide, crystalline tungsten oxide,crystalline praseodymium calcium manganese oxide, or other similarconductive oxide material. Other semiconductor materials and/orconductive oxide materials may be used.

For example, between about 10 angstroms to about 100 angstroms ofamorphous silicon may be deposited to form semiconductor material layer322, and between about 50 angstroms and about 150 angstroms ofcrystallized titanium oxide may be deposited over semiconductor materiallayer 322 to form conductive oxide material layer 324. Othersemiconductor materials such as amorphous tantalum nitride, amorphoustantalum silicon nitride, or other similar semiconductor material, otherconductive oxide materials such as crystalline zinc oxide, crystallinetungsten oxide, crystalline praseodymium calcium manganese oxide, orother similar conductive oxide material and/or other material layerthicknesses may be used.

An anisotropic etch is used to remove lateral portions of semiconductormaterial layer 322 and conductive oxide material 324, leaving onlysidewall portions of semiconductor material layer 322 and conductiveoxide material 324, resulting in the structure shown in FIGS. 4H1-4H2.

Next, first etch stop layer 316 is patterned and etched to expose topsurfaces of bit line select transistors Q₁₁-Q₃₁, resulting in thestructure shown in FIGS. 4I1-4I2.

Next, a second conductive material having a second work function φ2(e.g., titanium nitride, tantalum nitride, titanium carbide, tantalumcarbide, or other conductive material) is deposited over substrate 302,filling holes 422 and forming vertical bit lines LBL₁₁-LBL₃₃. In anembodiment, first work function φ1 of the first conductive material usedto form word lines WL₁₀, WL₁₁, . . . , WL₅₃ is greater than second workfunction φ2 of the second conductive material used to form vertical bitlines LBL₁₁-LBL₃₃. In an embodiment, first work function φ1 is greaterthan second work φ2 function by at least about 200 mV. The structure isthen planarized using chemical mechanical polishing or an etch-backprocess, resulting in the structure shown in FIGS. 4J1-4J2.

Thus, as described above, one embodiment of the disclosed technologyincludes a method that includes forming a dielectric material above asubstrate, forming a hole in the dielectric material, the hole disposedin a first direction, forming a word line layer above the substrate viathe hole, the word line layer disposed in a second directionperpendicular to the first direction, the word line layer including afirst conductive material having a first work function, forming anonvolatile memory material on a sidewall of the hole, the nonvolatilememory material including a semiconductor material layer and aconductive oxide material layer, forming a local bit line in the hole,the local bit line including a second conductive material having asecond work function, wherein the first work function is greater thanthe second work function, and forming a memory cell comprising thenonvolatile memory material at an intersection of the local bit line andthe word line layer.

One embodiment of the disclosed technology includes a method includingforming a sacrificial material layer above a substrate, etching thesacrificial material layer to form a row of sacrificial materialdisposed in a first direction, forming a dielectric material adjacentthe row of sacrificial material, forming a hole in the dielectricmaterial, the hole adjacent the row of sacrificial material and disposedin a second direction perpendicular to the first direction, replacingthe row of sacrificial material with a first conductive material to forma word line layer, the first conductive material including a first workfunction, forming a nonvolatile memory material on a sidewall of thehole, the nonvolatile memory material including a semiconductor materiallayer and a conductive oxide material layer, forming a local bit line inthe hole, the local bit line including a second conductive materialhaving a second work function, wherein the first work function isgreater than the second work function, and forming a memory cellincluding the nonvolatile memory material at an intersection of thelocal bit line and the word line layer.

One embodiment of the disclosed technology includes method of forming amonolithic three-dimensional memory array, the method including forminga stack of sacrificial material layers above a substrate; etching thestack of sacrificial material layers to form rows of sacrificialmaterial layers; forming a dielectric material between the rows ofsacrificial material layers; forming a plurality of holes in thedielectric material, the holes disposed between the rows of sacrificialmaterial layers; removing the rows of sacrificial material layers viathe plurality of holes to form a plurality of cavities; forming a firstconductive material in each of the cavities to form a plurality of wordline layers, the first conductive material having a first work function;forming a nonvolatile memory material on a sidewall of each of theplurality of holes, the nonvolatile memory material including asemiconductor material layer and a conductive oxide material layer;forming a plurality of local bit lines in the plurality of holes, thelocal bit lines including a second conductive material having a secondwork function, wherein the first work function is greater than thesecond work function, and forming an array of memory cells, each memorycell including the nonvolatile memory material at an intersection of oneof the plurality of local bit lines and one of the plurality of wordline layers.

For purposes of this document, each process associated with thedisclosed technology may be performed continuously and by one or morecomputing devices. Each step in a process may be performed by the sameor different computing devices as those used in other steps, and eachstep need not necessarily be performed by a single computing device.

For purposes of this document, reference in the specification to “anembodiment,” “one embodiment,” “some embodiments,” or “anotherembodiment” may be used to described different embodiments and do notnecessarily refer to the same embodiment.

For purposes of this document, a connection can be a direct connectionor an indirect connection (e.g., via another part).

For purposes of this document, the term “set” of objects may refer to a“set” of one or more of the objects.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims.

The invention claimed is:
 1. A method comprising: forming a dielectricmaterial above a substrate; forming a hole in the dielectric material,the hole disposed in a first direction; forming a word line layer abovethe substrate via the hole, the word line layer disposed in a seconddirection perpendicular to the first direction, the word line layercomprising a first conductive material having a first work function;forming a nonvolatile memory material on a sidewall of the hole, thenonvolatile memory material comprising a semiconductor material layerand a conductive oxide material layer; forming a local bit line in thehole, the local bit line comprising a second conductive material havinga second work function, wherein the first work function is greater thanthe second work function; and forming a memory cell comprising thenonvolatile memory material at an intersection of the local bit line andthe word line layer.
 2. The method of claim 1, further comprising:forming a plurality of word line layers above the substrate via thehole, each of the plurality of word line layers disposed in the seconddirection; and forming a plurality of memory cells comprising thenonvolatile memory material, each of the memory cells formed at anintersection of the local bit line and a corresponding one of the wordline layers.
 3. The method of claim 1, further comprising: forming aplurality of holes in the dielectric material, the plurality of holesdisposed in the second direction; forming the word line layer via theplurality of holes; forming a nonvolatile memory material on a sidewallof each of the holes; forming a plurality of local bit lines, each localbit line disposed in a corresponding one of the holes; and forming aplurality of memory cells comprising the nonvolatile memory material,each of the memory cells formed at an intersection of the word linelayer and a corresponding one of the local bit lines.
 4. The method ofclaim 1, further comprising forming a vertically-oriented transistorabove the substrate, and wherein forming the hole comprises forming thehole above the vertically-oriented transistor.
 5. The method of claim 1,further comprising: forming a global bit line above the substrate, theglobal bit line disposed in a third direction perpendicular to the firstdirection and the second direction; and forming a transistor between thevertical bit line and the global bit line.
 6. The method of claim 5,wherein the transistor comprises a vertical transistor.
 7. The method ofclaim 1, further comprising: forming a sacrificial material layer abovethe substrate; removing the sacrificial material layer to form a cavity;and forming the word line layer in the cavity.
 8. The method of claim 7,wherein removing the sacrificial material layer comprises etching thesacrificial material layer via the hole.
 9. The method of claim 1,wherein: the semiconductor material layer comprises one or more of thegroup of amorphous silicon, amorphous tantalum nitride, and amorphoustantalum silicon nitride; and the conductive oxide material layercomprises one or more of the group of crystalline titanium oxide,crystalline zinc oxide, crystalline tungsten oxide, and crystallinepraseodymium calcium manganese oxide.
 10. The method of claim 1,wherein: the first conductive material comprises one or more of thegroup of tungsten, tungsten nitride, cobalt, and ruthenium; and thesecond conductive material comprises one or more of the group oftitanium nitride, tantalum nitride, titanium carbide, and tantalumcarbide.
 11. A method comprising: forming a sacrificial material layerabove a substrate; etching the sacrificial material layer to form a rowof sacrificial material disposed in a first direction; forming adielectric material adjacent the row of sacrificial material; forming ahole in the dielectric material, the hole adjacent the row ofsacrificial material and disposed in a second direction perpendicular tothe first direction; replacing the row of sacrificial material with afirst conductive material to form a word line layer, the firstconductive material comprising a first work function; forming anonvolatile memory material on a sidewall of the hole, the nonvolatilememory material including a semiconductor material layer and aconductive oxide material layer; forming a local bit line in the hole,the local bit line including a second conductive material having asecond work function, wherein the first work function is greater thanthe second work function; and forming a memory cell comprising thenonvolatile memory material at an intersection of the local bit line andthe word line layer.
 12. The method of claim 11, wherein the sacrificialmaterial comprises one or more of the group of a nitride, a silicateglass and a semiconductor material.
 13. The method of claim 11, furthercomprising: forming a plurality of sacrificial material layers above thesubstrate; etching the plurality of sacrificial material layers to forma row of sacrificial material layers disposed in the first direction;replacing each of the sacrificial material layers with the firstconductive material to form a plurality of word line layers; and forminga plurality of memory cells comprising the nonvolatile memory material,each of the memory cells formed at an intersection of the local bit lineand a corresponding one of the word line layers.
 14. The method of claim11, further comprising: forming a plurality of holes in the dielectricmaterial, the plurality of holes disposed in the first direction, eachof the holes adjacent the row of sacrificial material; forming anonvolatile memory material on a sidewall of each of the holes; forminga plurality of local bit lines, each local bit line disposed in acorresponding one of the holes; and forming a plurality of memory cellscomprising the nonvolatile memory material, each of the memory cellsformed at an intersection of the word line layer and a corresponding oneof the local bit lines.
 15. The method of claim 11, wherein replacingthe sacrificial material layer comprises: removing the sacrificialmaterial layer to form a cavity; and forming the conductive material inthe cavity.
 16. The method of claim 15, wherein removing the sacrificialmaterial layer comprises etching the sacrificial material layer via thehole.
 17. The method of claim 11, wherein: the semiconductor materiallayer comprises one or more of the group of amorphous silicon, amorphoustantalum nitride, and amorphous tantalum silicon nitride; and theconductive oxide material layer comprises one or more of the group ofcrystalline titanium oxide, crystalline zinc oxide, crystalline tungstenoxide, and crystalline praseodymium calcium manganese oxide.
 18. Themethod of claim 11, wherein: the first conductive material comprises oneor more of the group of tungsten, tungsten nitride, cobalt, andruthenium; and the second conductive material comprises one or more ofthe group of titanium nitride, tantalum nitride, titanium carbide, andtantalum carbide.
 19. A method of forming a monolithic three-dimensionalmemory array, the method comprising: forming a stack of sacrificialmaterial layers above a substrate; etching the stack of sacrificialmaterial layers to form rows of sacrificial material layers; forming adielectric material between the rows of sacrificial material layers;forming a plurality of holes in the dielectric material, the holesdisposed between the rows of sacrificial material layers; removing therows of sacrificial material layers via the plurality of holes to form aplurality of cavities; forming a first conductive material in each ofthe cavities to form a plurality of word line layers, the firstconductive material having a first work function; forming a nonvolatilememory material on a sidewall of each of the plurality of holes, thenonvolatile memory material including a semiconductor material layer anda conductive oxide material layer; forming a plurality of local bitlines in the plurality of holes, the local bit lines including a secondconductive material having a second work function, wherein the firstwork function is greater than the second work function; and forming anarray of memory cells, each memory cell comprising the nonvolatilememory material at an intersection of one of the plurality of local bitlines and one of the plurality of word line layers.
 20. The method ofclaim 19, wherein: the semiconductor material layer comprises one ormore of the group of amorphous silicon, amorphous tantalum nitride, andamorphous tantalum silicon nitride; and the conductive oxide materiallayer comprises one or more of the group of crystalline titanium oxide,crystalline zinc oxide, crystalline tungsten oxide, and crystallinepraseodymium calcium manganese oxide.